Photoconductive layers and process for electrophotography



May 1966 G. SCHAUM ETAL 3,252,794

PHOTOCONDUCTIVE LAYERS AND PROCESS FOR ELECTROPHOTOGRAPHY Filed Nov. 2,1962 INVENTORS GUS MV SCHAUM, H/LDEGARD HAYDN, ANITA VON KON/G,

HEINRICH SEIBEFPT. BY

Cong 6% WA A T TORNE Y5 United States Patent Claims. (:L 961) Thisapplication is a continuation-impart of our application Serial No.696,032, filed November 13, 1957, now abandoned.

The present invention relates to photoconductive layers forelectrophotography.

For the production of photoconductive layers for electrophotography itis known to use certain inorganic or organic photo'conductive compounds.Examples of such compounds are sulphur, selenium, oxides, sulphides andselenides of zinc, cadmium, mercury, antimony, bismuth, and lead, andalso anthracene and anthraquinone. These substances are applied to anelectrophotographic plate, especially a plate consisting of metal oranother material, for example paper, or to a sheet or foil produced froma film-forming plastic. If necessary, the photoconductive substance canbe held dispersed in an electrically isolating, layer-forming binder.Such layers being produced by means of coating solutions in which thebinders are either dissolved or dispersed and in which theelectro-conductivesubstances are dispersed. Such photoconductive layersare for instance disclosed-in United States Patents Nos. 2,297,691;2,357,809; 2,727,807; 2,727,808; 2,735; 784; and 2,735,785, and inFrench Patents Nos. 1,113,- 933; 1,122,275; 1,125,235; and 1,136,146.

It has now been found that the electrophotographic layers with very goodproperties are obtained if organic benzoid compounds containing 4condensed aromatic rings of the following structures are used in suchelectrophotographic layers.

I. Benzanthrone II. Anthrapyrldine O III. Anthrapyridone Group V:

3,252,794 Patented May 24, 1966 IV. Anthrapyrimldine V. AnthrapyrlmldoneR represents hydrogen, lower alkyl up to 5 carbon atoms or aryl such asphenyl.

The electrophotographic efiicacy of the compound of the invention isbased on the cyclic structure identified above. The basic compounds suchas benzanthrone, anthrapyridine, anthrapyridone, anthrapyrimidine andanthrapyriinidone as well as substitution products thereof are suitablefor the instant purpose. In certain cases, substitution products may beadvantageous since certain secondary properties such-as solubility andthe like can be influenced by substituents.

Suitable substituents in particular of the fused benzene rings of thesaid structures are, for example, cyano, nitro, amino, halogen such aschlorine or bromine, hydroxyand amino-groups which are substituted withalkyl-, arylor acyl-groups.

The following compounds are, for example, suitable: Group I:

Benzanthrone 3-chlorbenzanthrone 3-brombenzanthrone 3-cyanobenzanthrone3,9-dichlorbenzanthrone 3,9-dibrombenzanthrone 3,9-dicyanobenzanthrone3-nitrobenzanthrone Z-methylbenzanthrone "Group II:

Anthrapyridine Group III:

Anthrapy-ridone N-methylanthr'apyridone 6-bromo-N-methylanthrapyridone6-amino-N-methylanthrapyridone 6-p-to1uidinb-N-methylanthrapyridoneGroup IV: I I

Anthrapyrimidine 6-aminoant-hrapyrimidine S-arninoanthrapyrimidineAnthrapyrimidone 6-p-toluidino-N-methylanthrapyrirnidoneN-rn'ethylanthrapyrirnidone The photoconductive compounds according tothe invention are advantageous over prior art because they are moresensitive to longer wavelength. It is furthermore possible to sensitizethese substances for still longer wavelengths by suitable compounds asthey are for instance disclosed in French Patent No. 1,125,235.

The coating solution which is used for the production of thephotoconductive layers comprises at least one of the aforementionedsubstances and a film-forming binding agent, the electrical specificresistance of which must be higher than that of the photoconductivesubstance and also higher than that of the layer support. The bestresults are obtained with binding agents the electrical specificresistance of which amounts to at least 10 ohms per centimeter.

Particularly suitable binding agents are silicone resins such asalkylpolysiloxanes and arylpolysiloxanes, especially pheny and methypolysiloxanes as they are disclosed in German Patents No. 853,351 and865,975.

Further suitable binding agents are for instance cellulose, celluloseesters, cellulose ethers, polyvinyl chloride, polyurethanes, polyesters,polyamides, polycarbonates with a base of di-(monohydroxyaryl)-alkanes,especially 4,4'-di(monohydroxyaryl)-alkanes according to German PatentsNos. 971,790 and 971,777. Photoconductive compounds can be present inthe electrophotographic layers, homogeneously in dissolved form orheterogeneously in a suspended form whereby the particle size of thesuspended photoconductor should be as small as possible.

The quantitative ratios between the photoconductive substances and thebinding agents may vary within wide limits. It is preferred to use thephotoconductive substance in amounts between 1 part for each 0.3 to 2parts by weight of binding agent which is the equivalent of 0.5 to 3.33parts by weight of the photoconductive substance to each part by weightof the binding agent, and amounts between about 5 and about 40 grams persquare meter of photoconductive layer. The solvent or solvent mixtureused for the production of electrophotographic layers containing aphotoconductive compound, heterogeneously dispersed therein must be goodsolvents for the binding agent but non-solvents or poor solvents for thephotoconductive compounds. Suitable layers of this type can be producedas follows: An organic solvent is used which dissolves both thephotoconductive substance and the binding agent layer former. Thissolution has added thereto another organic solvent in which the layerformer is soluble but the photoconductive substance is insoluble. Bythis means, the photoconductive compound is deposited in a state ofparticularly fine distribution, so that layers with a particularlysmooth surface are obtained.

As a support for the photoconductive layer there may be used paper ormetal plates, such as zinc, aluminium, or brass plates. Furthermore,thin foils of cellulose hydrate, cellulose esters or of polyamide comeinto question. In processing of the electrophotographic materials saidmaterial comprising a support of low electrical resistance and aphotoconducting insulating layer, coated thereon, is electrostaticallycharged in the dark, for instance, by means of a corona dischargedevice. The charged layer is then exposed with light through a negativephotographic film, positive film o-r mask or otherwise to a light image,to receive a latent electrostatic image. If the electrophotographicmaterial has been properly prepared, the charges leak off rapidly to thesupport in proportion to the intensity of light to which any given areais exposed. After such exposure, the latent electrostatic image can bedeveloped by any developing process known per se, for example, bydusting a developing powder whereby the powder adheres to the areaswhere the electrostating charges remain, forming thereby a powder imagecorresponding to the electrostatic image. Thereafter the powder imagecan be fixed by melting a developing powder or can be transferred to asheet of transfer material resulting in a positive or negative print asthe case may be.

The such light-conductive layers can also be produced from aqueousdispersions of photoconductive substances and binding agents, forinstance by the process disclosed in French Patent No. 1,136,146. Ifnecessary, the cast dispersions or emulsions are subjected to a finalcondensation or final polymerization by heat treatment at temperaturesof about -150 C. for about 2 to 30 minutes after being dried. Aqueousdispersions of polymeric substances, such as melamine-formaldehyde orurea-formaldehyde resins, xylene-formaldehyde resins, polymers based onvinyl chloride, vinylidene chloride, vinyl ethers, acrylic esters,methacrylic esters, acrylic amides, such for example as methacrylicamide, aromatic vinyl compounds, such for example as styrene,isoolefins, such for example as isobutylene, copolymers based on theaforementioned compounds, such for example as copolymers of vinylchloride and butyl acrylate vinylidene chloride and butyl acrylatesynthetic elastomers, such for example as copolymers of butadiene andstyrene, butadiene and acrylonitrile, and also copolymers of dienes witha preponderant proportion of styrene and/ or acrylonitrile or othervinyl compounds, as well as polyamides, polyurethanes, polyesters ofpolycarboxylic acids and polyhydric alcohols, polycarbonates, celluloseester, rubber and the like, are for example employed.

Example 1 g. of a 30% solution in ethyl acetate of a copolymer of vinylchloride and acrylic acid ester 100 cc. of ethyl acetate 40 g. ofbenzanthrone are mixed and ground for several hours in a ball mill. Themixture is then cast on a support, for example, a paper support anddried. The resulting electrophotographic layer has a very lowconductivity in the dark or visible and a high conductivity if exposedto ultraviolet light.

Example 2 64 g. of a 60% solution in toluene of a silicone resin asdisclosed in Example 1 of German Patent No. 853,351,

97 cc. of toluene and 40 g. of 3,9-dibrom benzanthrone are mixed andthoroughly ground for several hours in a ball mill. The mixture isfurther treated as described in Example 1.

Example 3 64 g. of a toluene solution of a silicone resin as describedin Example 2,

97 cc. of toluene and 20 g. of anthrapyridone are mixed and thoroughlyground in a ball mill. The mixture is cast on a paper support and dried.An excellent smooth layer, which is only very slightly yellow colored isobtained. The layer is sensitive to the visible range of the spectrum.

By usual processing which comprises charging by corona dischargeexposing and developing the resulting electrostatic latent image, adeveloping powder image is obtained having excellent detail and quality.

Example 4 An electrophotographic material is produced as described inExample 3 with the exception that anthrapyridone is replaced with 22 g.of 8-aminoanthrapyridine. The resulting layers show a very lowconductivity in the dark and a high conductivity if exposed toultraviolet and blue light of the spectrum.

Example 5 An electrophotographic material is produced as described inExample 3 with the exception that anthrapyridone is replaced byanthrapyrimidine. The resulting layer is very smooth, only slightlyyellow colored and sensitive to the visible range of the spectrum.Similar results are obtained if anthrapyrimidine is replaced withequivalent amounts of anthrapyrimidone, anthrapyridine,N-methylanthrapyrimidone, 4-brom-N-methylanthrapyridone, 4-amino-N-methylanthrapyridone or 4-p-toluidino-N-methylanthrapyridone.

Example 6 100 g. of a 30% solution in ethyl acetate of a copolymer ofphenyl chloride and acrylic acid methyl ester,

100 cc. of ethyl acetate and 40 g. of 6-aminoanthrapyrimidine are mixedand further treated as described in Example 1.

The resulting photographic layer has excellent electrophotographicproperties.

Example 7 100 cc. of a 2.5% solution of a 2.5% solution in acetone ofcellulose acetate,

100 cc. of acetone and 22 g. of 6-p-toluidino-N-methylanthrapyridone areheated while stirring up to a temperature of about 50 C., whereby thephotoconductive compound is partially dissolved. The hot mixture is caston a paper support and slowly dried.

The resulting electrophotographic layer has good properties and can beprocessed in the usual manner.

Example 8 30 g. of 3-nitrobenzanthrone are added to a mixture of 100 cc.of a 30% aqueous dispersion of a heat-hardenable melamine-formaldehydecondensation product and 80 cc. of water.

The mixture is treated for 2 hours in a ball mill and thereafter coatedon a paper support. After drying at room temperature the material isheated to 80 C. for 15 minutes.

By usual processing which comprises charging by corona dischargeexposing and developing the resulting electrostatic latent image, adeveloping powder image is obtained having excellent detail and quality.

Example 9 In this example the electrophotographic properties of severalcompounds according to the invention are compared with knownphotoconductive compounds.

The sensitivity of an electrophotographic material depends essentiallyon the difference between the photoconductivity and thedark-conductivity. The more electrostatic charge drains oiT by a givenexposure the higher shall be the contrast respectively the quality ofthe resulting image. The graphs attached thereto present the drainingoff of the charge by exposure as a function of time. The steeper is theslope of the curve the more suitable is the compound forelectrophotographic purposes. The axis of abscissa shows the relativecharge per unit area in logarhythmic scale in arbitrary units and theaxis of ordinate the time in arbitrary units.

The electrophotographic material to be tested where prepared accordingto Example 2 by mixing each 40 g. of photoconductive compounds with 64g. of a solvent of the silicone resin described in that example. Witheach compound the processing, charging and exposure used identicaltechnic.

The curves in FIGURES 1 and 2 represent the following photoconductivecompounds:

FIGURE 1: Known photoconductive compounds. Curve 1: Anthracene.

Curve 2: p-Terphenyl.

Curve 3: Anthraquinone.

FIGURE 2: Photoconducters according to the invention. Curve 1:3,9-dibromobenzanthrone.

Curve 2: 6-aminoanthrapyrimidine.

Curve 3: Anthrapyrimidone.

We claim:

1. An electrophotographic material comprising a photoconductinginsulating layer carried on a support, said photoconducting layer beingcomposed of a dielectric film forming organic resin and aphotoconductive organic compound dispersed therein, said photoconductiveorganic compound containing 4 condensed aromatic rings selected fromthose having the following structure:

and

wherein R stands for a radical selected from the group consisting ofhydrogen, lower alkyl having up to 5 carbon atoms and phenyl, saidphotoconductive organic compound being employed in an amount of 1 partper 0.3-2 parts by weight of film forming organic resin and in an amountof 5 to 40 g./ sq. meter of surface, the film forming organic resinhaving an electrical specific resistance of at least 10 ohms percentimeter which specific resistance is higher than the specificresistance of the support.

2. An electrophotographic material according to claim 1, wherein saidphotoconducting insulating layer contains as a photoconductive compoundbenzanthrone.

3. An electrophotographic material according to claim 1, wherein saidphotoconducting insulating layer contains as a photoconductive compound3.9-dibrombenzanthrone.

4. An electrophotographic material according to claim 1, wherein saidphotoconducting insulating layer contains as a photoconductive compoundanthrapyridone.

5. An electrophotographic material according to claim 1, wherein saidphotoconducting insulating layer contains as a photoconductive compound8-aminoanthrapyrirnidine.

6. An electrophotographic material according to claim 1, wherein saidphotoconducting insulating layer contains as a photoconductive compoundanthrapyrimidine.

7. An electrophotographic material according to claim 1, wherein saidphotoconducting insulating layer contains as a photoconductive compound6-aminoanthrapyrirnidine.

8. An electrophotographic reproduction process which comprises exposingan electrostatically charged supported photoconductive insulating layerto light under a master to discharge the layer in proportion to theintensity of light to which any given area is exposed and developing theresulting latent electrostatic image with an electrophotographicdeveloper, the said photoconductive layer comprising as aphotoconductive compound an organic benzonoid compound containing 4condensed aromatic rings selected from those having the followingstructures:

the said photoconductor being employed in an amount of 1 part per 0.3-2parts by weight of binding agent, and in an amount of 5 to 40 g./ sq.meter of surface.

9. A process according to claim 8, wherein said photoconductive layercontains as a photoconductive compound benzanthrone.

10. A process according to claim 8, wherein said photoconductive layercontains as a photoconductive compound 3.9-dibrombenzanthrone.

11. A process according to claim 8, wherein said photoconductive layercontains as a photoconductive compound anthrapyridone.

12. A process according to claim 8, wherein said photoconductive layercontains as a photoconductive compound 8-arninoanthrapyrimidine.

13. A process according to claim 8, wherein said photoconductive layercontains as a photoconductive compound anthrapyrimidine.

14. A process according to claim 8, wherein said photoconductive layercontains as a photoconductive compound 6-aminoanthrapyrimidine.

15. A photoconductive sheet material for electrophotography consistingof a photoconductive substance comprising a silicone resin binding agenthaving dispersed therein as a photoconductive insulating substanceanthrapyrimidone, the photoconductive layer being cast upon anelectrically conductive support, the anthrapyrimidone being employed inan amount of 1 part per 0.32 parts by Weight of silicone resin, and inan amount of 5 to 40 g./ sq. meter of photoconductive layer.

References Cited by the Examiner UNITED STATES PATENTS 2,123,245 7/ 1938Koeberle et a1 260-261 2,138,381 11/1938 Koeberle et al 260261 2,629,9563/ 1953 Switzer 84 X 2,663,636 12/1953 Middleton 96--1 2,692,178 10/1954Grandadam 961 2,759,939 8/1956 Bucheler 839 3,000,735 9/1961 Gunning etal 961 OTHER REFERENCES Inokuchi: Chemical Society of Japan Bulletin,27:22-27, 1954.

NORMAN G. TORCHIN, Primary Examiner.

ALEXANDER D. RICCI, C. E. VAN HORN, Examiners.

1. AN ELECTROPHOTOGRAPHIC MATERIAL COMPRISING A PHOTOCONDUCTINGINSULATING LAYER CARRIED ON A SUPPORT, SAID PHOTOCONDUCTING LAYER BEINGCOMPOSED OF A DIELECTRIC FILM FORMING ORGANIC RESIN AND APHOTOCONDUCTIVE ORGANIC COMPOUND DISPERSED THEREIN, SAID PHOTOCONDUCTIVEORGANIC COMPOUND CONTAINING 4 CONDENSED AROMATIC RINGS SELECTED FROMTHOSE HAVING THE FOLLOWING STRUCTURE: